1. Field of the Invention
The present invention relates to a fuel-injection control system for an internal combustion engine, and more particularly, to such a fuel-injection control system which is adapted to enable an engine to operate in a proper manner under the action of a backup means when an intake-air sensor for detecting the loading condition of the engine has failed.
2. Description of the Prior Art
In the past, in order to improve the output power and emission control of automotive gasoline engines, many so-called electronically-controlled fuel injection systems have been widely developed and reduced into practice which have, as a major component, an electronic control device for controlling the opening time of a fuel injection valve(s), which is adapted to inject pressurized fuel into the engine, on the basis of the information obtained from an intake-air sensor for detecting the amount of intake air sucked into the engine, and other various sensors. In cases where the intake-air sensor, the most important of various components of such a fuel-injection system, has failed, the operation of the engine becomes impossible so that the vehicle equipped with the engine loses its normal vehicular function. Therefore, some backup means is required to avoid such a situation.
In order to meet such requirements, a fuel-injection control system, as illustrated in FIG. 1, was proposed which is of a multiple point type having an intake-air sensor. In FIG. 1, there is schematically shown the general arrangement of an automotive internal combustion engine which includes an engine proper 1, an intake-air sensor 2 in the form of a flow-rate sensor for detecting the amount or flow rate of intake air sucked into the engine proper 1, a plurality of fuel injection valves 3a through 3d disposed in an intake passage 6 at a location downstream of a throttle valve 7, an engine RPM sensor 4 adapted to pick up engine revolution signals for generating an output signal representative of the RPMs of the engine proper 1, a temperature sensor 5 adapted to generate an output signal representative of the temperature of an engine coolant 16, a throttle valve 7 adapted to regulate the amount or flow rate of intake air sucked into the engine proper 1, a throttle-opening sensor 8 adapted to generate an output signal representative of the opening degree of the throttle valve 7, the throttle-opening sensor 8 being formed, for example, of a variable resistor and adapted to be used for correcting the amount of fuel injected from the injection valves 3a through 3d during acceleration or engine operation with the throttle valve 7 fully opened, a bypass conduit 9 for connecting between an upstream portion and a downstream portion of the intake passage 6 with respect to the throttle valve 7 for bypassing a part of intake air in the intake passage 6 across the throttle valve 7, a bypass valve 10 disposed in the bypass conduit 9 for controlling the flow rate of intake air flowing therethrough, a control unit 11 including an input interface 100, a microprocessor 101, a ROM 102, a RAM 103 and an output interface 104, the control unit 11 being adapted to receive output signals from the intake-air sensor 2, the engine RPM sensor 4, the temperature sensor 5, and the throttle-opening sensor 8 for controlling the operations of the fuel injection valves 3a through 3d in an appropriate manner, and a thermo-element 15 adapted to control the operation of the bypass valve 10 in accordance with the temperature of the engine coolant 16.
The operation of the above-described fuel-injection control system is well known in the art and hence only the important points will be briefly described in the following. First, the output signal of the intake-air sensor 2 representative of the detected amount of intake air sucked into the engine proper 1, the output pulse of the engine RPM sensor 4 representative of the detected engine RPMs, and the output signal of the temperature sensor 5 representative of the operating temperature of the engine proper 1 are input, as input information, to the input interface 100 of the control unit 11. Then, the microprocessor 101 calculates a pulse width and a pulse cycle to be fed to the fuel injection valves 3a through 3d in accordance with the operation processing program stored in the ROM 102, and the pulse width and the pulse cycle thus calculated are amplified by the output interface 104 and fed to the respective fuel injection valves 3a through 3d so as to operate these valves in an appropriate manner. Though not illustrated, the fuel injection valves 3a through 3d are supplied with pressurized fuel by a fuel pressurizing means (not shown). The flow-rate-type intake-air sensor 2 is generally of a vane type, a hot wire type, or a Karman type, but in place of such a flow-rate sensor, a pressure sensor may be employed which serves to detect the pressure of intake air sucked into the engine proper 1.
With the above fuel-injection control system, however, the engine will become inoperative should the intake-air sensor 2 fail. To avoid such a situation, various backup measures have been proposed. According to the most popular one of these backup measures, different fixed pulse widths are set for driving the fuel injection valves 3a through 3d at the time of the idling operation and the non-idling operation of the engine, respectively. In this measure, the actual air/fuel ratio of a mixture fed to the engine proper 1 becomes a desired value only at a certain opening degree of the throttle valve 7, but in almost all the remaining opening range of the throttle valve 7, the air/fuel ratio becomes too rich or lean, thus substantially impairing the operational performance of the engine.
In order to compensate for the above defect, an improved measure has recently been taken in which the amount of fuel to be injected into the engine proper 1 is controlled by the use of the output signal of the throttle-opening sensor 8 representative of the opening degree of the throttle valve 7 and the output signal of the engine RPM sensor 4 representative of the engine RPMs. The operation of this fuel injection control system will now be described with reference to a flow chart illustrated in FIG. 2. In this figure, at step 100, it is determined whether or not the intake-air sensor 2 has failed. This process differs according to the type of intake-air sensor 2 employed. For example, in case of a hot-wire type sensor, the characteristic of the output voltage with respect to the amount of intake air is represented by a curve (i.e., a 4th-power-root curve) shown in FIG. 4, and the output voltage (V) actually used is in the range from V.sub.1 to V.sub.2 so that, if V.sub.1 &gt;V&gt;V.sub.2, it can be determined that the sensor 2 is in a failed state. On the other hand, in case of a Karman type sensor, the frequency of the sensor output is in direct proportion to the intake-air amount, as illustrated in FIG. 5, and the output frequency f actually used is in the range from f.sub.1 to f.sub.2 so that, if f.sub.1 &gt;f&gt;f.sub.2, it can be determined that the sensor 2 is in a failed state. In another measure, the output range of the intake-air sensor 2 is predetermined in accordance with the opening degree of the throttle valve 7 and the RPMs of the engine so that, if the sensor output is out of the predetermined range, it is determined that the sensor 2 has failed. If the sensor 2 is determined to be normal as a result of step 100 by any one of the above-described measures, normal operation processing is effected at step 101. On the other hand, if the sensor 2 is determined to have failed, the output signal of the throttle-opening sensor 8 representative of the detected opening degree (.theta.) of the throttle valve 7 is read out at step 102, and the output signal of the engine RPM sensor 4 representative of the detected engine RPMs (Ne) is then read out at step 103. Subsequently, at step 104, a basic pulse width (.tau..sub.0) for driving the fuel injection valves 3a through 3d is determined in accordance with the detected throttle opening (.theta.) and the detected engine RPMs (Ne). The basic pulse width (.tau..sub.0) is prestored, as a two-dimensional map corresponding to the engine RPMs (Ne) and the throttle opening (.theta.), in the ROM 102 as illustrated in FIG. 3. Specifically, the actual basic pulse width (.tau..sub.0) is determined through an interpolating operation or calculation by using plural points which are read out from the two-dimensional map, and which are the nearest to the actual (.theta.) and (Ne) detected. At step 105, the basic pulse width (.tau..sub.0) thus obtained is corrected by a correction coefficient (Kc) which is determined on the basis of the output signal from the temperature sensor 5 and a fuel-correction signal issued upon acceleration or deceleration of the engine. Accordingly, by repeating the above-described operations, the amount of fuel injected from the fuel injectors 3a through 3d into the engine proper 1 can be controlled to an appropriate level in accordance with the opening degree of the throttle valve 7 even if the intake-air sensor 2 has failed, thereby enabling the backup operation of the engine.
However, the above-described conventional fuel-injection control system has the following problems. Specifically, the pulse width of the fuel injection valves 3a through 3d for controlling the air/fuel ratio of the mixture is determined from the opening degree of the throttle valve 7 and the engine RPMs, so that, when the opening degree of the bypass valve 10 in the bypass conduit 9 is varied during the fast-idling operation (warm-up operation) of the engine by the thermo-element 15, the stroke of which changes in response to the temperature of the engine coolant 16, the amount of intake air actually sucked into the engine proper 1 is equal to the sum of the amount of intake air flowing through the main intake passage 6, regulated by the throttle valve 7, and the amount of intake air flowing through the bypass conduit 9, regulated by the bypass valve 10. For this reason, the opening degree of the throttle valve 7 does not correspond to the actual amount of intake air, and hence there will be a great error in the air/fuel ratio of the mixture, thus impairing the proper operation of the engine.